The 3rd Generation Partnership Project, 3GPP, is responsible for the standardization of the Universal Mobile Telecommunication System, UMTS, and Long Term Evolution, LTE. The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network, E-UTRAN. LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink and in the uplink. In order to support high data rates, LTE allows for a system bandwidth of 20 MHz, or up to 100 MHz when carrier aggregation is employed, and LTE will continue to evolve. In parallel to the LTE evolution, a new generation of cellular technology is being developed, a 5th generation system, 5G. One of the tasks for 5G is to improve throughput and capacity compared to LTE. This is in part to be achieved by increasing the sampling rate and bandwidth per carrier. 5G is also including use of higher carrier frequencies i.e., above 6 GHz.
In an UTRAN and an E-UTRAN, a User Equipment, UE, or a wireless device is wirelessly connected to a Radio Base Station, RBS, commonly referred to as a NodeB, NB, in UMTS, and as an evolved NodeB, eNodeB or eNB, in LTE. A Radio Base Station, RBS, access node, AN, or access point, AP, is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. In Wireless Local Area Network, WLAN, systems the wireless device is also denoted as a Station, STA.
Future communications networks are expected to use advanced antenna systems to a large extent. With such antennas, signals will be transmitted in narrow transmission beams to increase signal strength in some directions, and/or to reduce interference in other directions. The high frequencies and reliance of beamforming makes it challenging to maintain a reliable radio link. A narrow beam can quickly be lost—in particular when combined with poor diffraction properties. Hence, beamforming based high-frequency radio access technologies are more susceptible to sudden changes in link quality or even loss of coverage, which may lead to significant delays and signaling until the wireless device can recover and find coverage again.
In LTE, some downlink reference signals are broadcasted in an always-on manner and over the full bandwidth, regardless of the presence or position of UEs in the system. These signals are called cell specific reference signals, CRS. A user equipment, UE, receiving the reference signal can measure the quality of neighbor cells for mobility purposes. Applying such continuous transmission of reference signals in all individual transmission beams in such a future cellular communications network may consume resources available for data, and generate a lot of interference in neighboring cells. Continuous transmissions also cause high energy consumption in the radio access points.
One option for reporting the measurements, also known as mobility reference signal measurements, from the UE back to the NW is to use RRC signaling. The information is aggregated at the L3 level and long report lengths may be supported that allow conveying information about many detected candidate link identities and their estimated signal strengths or qualities. Upon the reception of these measurement reports the network is capable of taking handover decisions, either to keep the UE in the cell, move it to another cell within the same frequency, another frequency and/or RAT. The link switch decision may then be taken by the NW considering a combination of link quality and network status parameters.
However another option for handling radio link with non-RRC reporting is also being investigated based on physical layer, L1-based, reporting using suitable uplink signals, e.g., Uplink Synchronization Sequences, USS, a locally unique signal carrying a synchronization pilot and an identity. Alternatively, the L1-based reporting may use a physical random access channel (PRACH) preamble. The UE may be configured to send the USS to the target to indicate e.g. the best of a set of candidate DL beams through a preconfigured USS sequence. The UE is configured for performing mobility measurements of multiple candidate beams and, upon detecting the strongest beam; it should directly access the node it originates from. This is done by sending an USS whose uplink, UL, resources (Time/Frequency slot, sequence) are associated with the downlink, DL, beam; the association is preconfigured by the network. The receiving AN, e.g. the target AN, reserves UL resources for USS detection ahead of time, which allows the report to be conveyed with minimal latency. Upon receiving a USS report, an AN, e.g. the target AN, may signal in the DL the new serving link to allow sync parameter updates and other possible configuration changes at the UE. This L1-based method is a quicker way to access a target beam in scenarios where the SINR of the serving link can quickly drop due to shadowing and provides for an improvement in terms of robustness and latency, due to the fact that the UE sends the reporting directly to the target while in the RRC-based method the UE needs to send the measurements to the source, wait for a decision from the source and eventually receive an RRC Connection Re-configuration from the serving link which might be under bad radio conditions.
RRC-based measurement reporting allows conveying reliable, rich measurement information from the UE to the network, but often with considerable signaling overhead and potentially high latency. Only using RRC signaling for mobility decisions, as in LTE, may create problems in some scenarios envisioned for the New Radio, NR, in 5G, such as when the UE is using a high gain beamforming in higher frequencies. There, the link quality provided by a beam can drop very quickly due to the aggressive shadowing effect that does not exist in frequencies when LTE is deployed and the narrow coverage of the beam. RRC signaling may therefore be unsuitable in scenarios in beam-formed systems where link quality diminishes rapidly outside the best coverage areas. The alternative approach, based on USS-reporting, may create problems in other scenarios and may entail complexity and resource drawbacks. In USS-based reporting, the UE can convey only a single link report at a time. Consequently, the beam switch or mobility procedure effectively becomes UE-controlled, since the best link detection and reporting from the UE determines the target link. Accordingly, there is a need to improve measurement reporting in support of mobility procedures.